A type of feldspar with chemical formula ranging continuously from AlSi3NaO8 (albite) to Al2Si2CaO8 (anorthite). Plagioclase feldspar has a complicated phase diagram with several different ordered states. Anorthite's low temperature ordered state is an alternating pattern of Al and Si atoms, but albite has a different ordering pattern, which cannot be created from anorthite's by replacing Al atoms by Si. Albite also has a displacive phase transition - this is a phase transition where the atoms do not swap places but the shape of the unit cell changes. In between albite and anorthite, there are more complicated ordered states known as incommensurate structures.